The present invention relates to a rubber composition suitable for use as a tire tread rubber, and more particularly to a rubber composition capable of giving an improved abrasion resistance to tire tread while retaining its low heat build-up.
As a result of the enhancement in the performances of automobiles in the recent years, more improved high-speed traveling performances, stability performances and durability performances have come to be waited for. Thus, it is currently important problem to improve the characteristic properties of tire tread with respect to these performances, and particularly to improve abrasion resistance without increasing its in heat build-up. Further, in order to cope with the social demands such as saving of resources, energies, etc., low fuel consumption tires are being studied energetically. This problem also makes it indispensable to give the rubber constituting a tire tread a high resilience and thereby lower its heat build-up.
Generally speaking, the performances of rubber, such as abrasion resistance, heat build-up and the like, are largely dependent on the characteristic properties of the carbon black to be compounded thereinto. It is known that these characteristics of rubber have an antinomic character.
For example, in lowering the heat build-up of a tire to thereby make a low fuel consumption tire, it is effective to compound a small quantity of a carbon black of relatively large particle size into a rubber. However, if the quantity of carbon black is small, some characteristic properties of tire, such as abrasion resistance, braking performance on wet road surface and the like, are deteriorated unavoidably.
When a tire tread is required to have a high abrasion resistance under severe traveling conditions, it is effective to compound a carbon black having as small a size as possible and a large structure. However, such of carbon black enhances the heat build-up of a rubber composition, so that it has the danger of promoting, for example, the breakage of the inner structure of a tire due to the accumulation of heat in the course of traveling and the aging of the materials constituting the tire.
Thus, there has been proposed a number of techniques simultaneously aiming at an improvement in abrasion resistance and a reduction of heat build-up by microscopically studying the fundamental properties of carbon black, such as particle size (specific surface area), structure, etc., and other properties, screening out a carbon black of specified properties, and compounding such a carbon black into a rubber.
For example, regarding the carbon black of N 339 class frequently used as a tire tread for passenger cars and light-weight trucks, the following proposals have been made.
(1) A carbon black for rubber formulation, wherein the nitrogen adsorption specific surface area (N.sub.2 SA) is 60 m.sup.2 /g or above, the dibutyl phthalate absorption number (DBP) of a compressed sample is not lower than 112 ml/100 g, and the Stokes mode diameter and its distribution of carbon black aggregate are not smaller than specified values (Japanese patent application Kokoku publication No. 1-53978).
(2) A rubber composition containing a carbon black, wherein the N.sub.2 SA of the carbon black is 60 m.sup.2 /g or above, the DBP is 108 ml/100 g or above, the true specific gravity per unit specific surface area is preset in a specified range much lower than that of known carbon blacks, and the tinting strength and the width of distribution per aggregate mode diameter are so controlled as not to drop below specified values (Japanese patent application Kokai publication No. 59-140241).
(3) A carbon black for rubber formulation, wherein the N.sub.2 SA is 65-84 m.sup.2 /g, the ratio of N.sub.2 SA to IA (iodine adsorption number) is in the range of 1.10-1.35, and a value defined by a formula involving the DBP of a compressed sample, blackness, IA and Stokes mode diameter of aggregate particles as variables i preset so as not to drop below a specified value (Japanese patent application Kokai publication No. 63-225639).
(4) A carbon black for rubber formulation, wherein the N.sub.2 SA is 75-105 m.sup.2 g, the DBP of compressed sample is 110 ml/100 g or greater, the true specific gravity per unit specific surface area is preset so as to be lower than that of known carbon blacks, and the intraaggregate pore diameter and the width of distribution per aggregate mode diameter are retained at values not smaller than specified values (Japanese patent application Kokai publication No. 1-201367).
Regarding carbon blacks of N 220 class and N 110 class used as a component of large-sized tire treads of trucks and buses, the following improvements and proposals have been made.
(5) A rubber composition containing a carbon black wherein the carbon black has N.sub.2 SA of 100-200 m.sup.2 /g and the Stokes mode diameter distribution of aggregate is broad as compared with its particle diameter (Japanese patent application Kokai publication No. 63-112638).
(6) A rubber composition containing a carbon black, wherein the carbon black has N.sub.2 SA of 70-185 m.sup.2 /g and the Stokes mode diameter distribution of aggregate has two maximum points in specified ranges (Japanese patent application Kokai publication No. 63-179941).
(7) A rubber composition containing a carbon black, wherein the carbon black has N.sub.2 SA of 110-155 m.sup.2 /g and a value defined by a formula involving DBP, DBP of a compressed sample, blackness, N.sub.2 SA and IA as variables is in a specified range (Japanese patent application Kokai publication No. 63-297439).
In spite of the many proposals mentioned above, however, an improved performance is expected of low fuel consumption tires currently is sought. That is, it is earnestly desired to develop a rubber composition of low heat build-up which exhibits an excellent abrasion resistance while retaining a high level of resilience.